Yes, the Guardian Flyer is overpowered. Its a specialized ship. A player based boss ship.
Elite play requires elite ships. The Guardian Flyer wouldnt be overpowered.
Hell, I flew a Navy Vexor worth 125 million ISK that survived for like ten minutes against T2 frigs. I lost the NVex but the point is, the Guardian Flyer would be nearly impossible for a 10 ship fleet to defeat, 20 ships, maybe and 30 very possibly.
The Gaurdian Flyer is meant to be a story similar to Legion where angry peasants come out to fight the vengeful angels.
When in certain range of a Guardian Flyer, 50 km, the Guardian Flyer has the ability to cancel out sec status hits, basically Null Sec. But at the same time, any ship can attack the Flyer without a sec status penalty while in High or Low.